1. Field of the Invention
The invention relates to the configuration and automatic control of an audio filter designed to remove unwanted interference of a single unknown constant or slowly varying frequency from a speech signal, or other desired signal of rapidly varying frequency, and to methods and apparatuses to remove unwanted interference of rapidly varying frequency and amplitude from a desired signal of constant or slowly varying frequency.
2. Description of Prior Art
The presence of an unwanted interfering signal superimposed on an audio signal of varying frequency is a major problem in the reception of single sideband transmissions, where a constant frequency interference is the result of a continuous wave transmission on a nearby frequency. A similar form of interference can occur when data and audio signals are mixed during transmission by wire or radio. A third source of constant frequency interference is observed in the oscillation caused by unexpected audio feedback such as in a public address system.
Heretofore a wide variety of apparatuses have been proposed and implemented to remove such unwanted interference. The most common attempted solution to such interference in a single sideband receiver is the introduction of a manually tuned notch filter in the intermediate frequency section of the receiver to tune out such interference. This solution has the disadvantage of being difficult to operate and having a wider notch than the optimal width. As a result, degradation of the desired signal is noted when the notch filter is in operation. In addition, manual tuning cannot be used in the case of interference which drifts randomly in frequency.
Similarly, audio notch filters which are manually tuned have been available for some time. Such filters are difficult to use because they must be adjusted manually by the operator. The need for manual adjustment requires that the notch width be wider than the optimum frequency in order to make the tuning practical. In addition, a manually tuned audio notch filter will not track interference of a varying frequency.
A previous automatic band rejection filter disclosed in U.S. Pat. No. 3,562,675 issued to Urell is an RF filter which uses an automatically tuned bridge T filter. The method applied is only appropriate for a narrow frequency range such as the separation of LORAN signals from adjacent channel interference, and requires high precision in the selection of components for proper operation which can make manufacturing difficult.
The applicant is also aware of an automatic notch filter presently marketed in the United Kingdom by Datong Electronics Ltd. The present invention differs significantly from such filter and is not anticipated thereby.